Inkjet printing is typically done by either drop-on-demand or continuous inkjet printing. In drop-on-demand inkjet printing ink drops are ejected onto a recording medium using a drop ejector including a pressurization actuator (thermal or piezoelectric, for example). Selective activation of the actuator causes the formation and ejection of a flying ink drop that crosses the space between the printhead and the recording medium and strikes the recording medium. The formation of printed images is achieved by controlling the individual formation of ink drops, as is required to create the desired image.
Motion of the recording medium relative to the printhead during drop ejection can consist of keeping the printhead stationary and advancing the recording medium past the printhead while the drops are ejected, or alternatively keeping the recording medium stationary and moving the printhead. The former architecture is appropriate if the drop ejector array on the printhead can address the entire region of interest across the width of the recording medium. Such printheads are sometimes called pagewidth printheads. A second type of printer architecture is the carriage printer, where the printhead drop ejector array is somewhat smaller than the extent of the region of interest for printing on the recording medium and the printhead is mounted on a carriage. In a carriage printer, the recording medium is advanced a given distance along a medium advance direction and then stopped. While the recording medium is stopped, the printhead carriage is moved in a carriage scan direction that is substantially perpendicular to the medium advance direction as the drops are ejected from the nozzles. After the carriage-mounted printhead has printed a swath of the image while traversing the print medium, the recording medium is advanced; the carriage direction of motion is reversed; and the image is formed swath by swath.
A drop ejector in a drop-on-demand inkjet printhead includes a pressure chamber having an ink inlet for providing ink to the pressure chamber, and a nozzle for jetting drops out of the chamber. Two side-by-side drop ejectors are shown in prior art FIG. 1 (adapted from U.S. Pat. No. 7,163,278) as an example of a conventional thermal inkjet drop-on-demand drop ejector configuration. Partition walls 20 are formed on a base plate 10 and define pressure chambers 22. A nozzle plate 30 is formed on the partition walls 20 and includes nozzles 32 (also called orifices herein), each nozzle 32 being disposed over a corresponding pressure chamber 22. The exterior surface of a nozzle plate 30 is called a nozzle face 114 herein. Ink enters pressure chambers 22 by first going through an opening in base plate 10, or around an edge of base plate 10, and then through ink inlets 24, as indicated by the arrows in FIG. 1. A heating element 35, which functions as the actuator, is formed on the surface of the base plate 10 within each pressure chamber 22. Heating element 35 is configured to selectively pressurize the pressure chamber 22 by rapid boiling of a portion of the ink in order to eject drops of ink through the nozzle 32 when an energizing pulse of appropriate amplitude and duration is provided.
During the printing process ink residue can collect on the nozzle face and within the nozzles and cause total or partial blockage of nozzles that can result in missing drops, small drops or misdirected drops of ink, thereby degrading print quality. To overcome this, a maintenance station is commonly used in order to clean the nozzles and to slow the evaporation of the volatile components of the ink. Maintenance stations typically include capability for exerting a pressure differential at the nozzle face to withdraw ink from the nozzles in order to prime the nozzles and remove blockages due to dried or viscous ink, air bubbles or particulates. While nozzle priming is effective in cleaning, it generally uses excessive amounts of ink and preferably should only be done infrequently. Periodic ejection of ink droplets, sometimes called spitting, while the printhead is at the maintenance station uses relatively small amounts of ink and is effective for removing some viscous ink plugs and some dried ink, but it is not effective in removing larger or more tenaciously adhering obstructions.
Many inkjet printing systems have maintenance stations that use wiping of the nozzle face to remove ink residue and other debris. Wipers are typically made of elastomeric materials for rubbing or soft absorbent materials for blotting. Over an extended period of time, wiping of the nozzles can cause damage to the nozzle face. Even though the wipers themselves may be soft, they can drag particulates across the nozzle face resulting in abrasion. For nozzle faces having an ink repellent coating, extended wiping can change the wettability of the nozzle face. Over a period of time the damage to the nozzle face can cause permanent damage that degrades print quality to the extent that the printhead needs to be replaced. Furthermore, wiping can smear ink residue or particulates into the nozzles, which can cause nozzle clogging or jet misdirection.
Developments within the inkjet printing industry have increased the importance of maintenance that is effective in cleaning nozzles without damaging the nozzle face. One development is the increasing use of inks that have more desirable printing characteristics on the print medium. An example is waterfast pigment-based inks. Pigments are not soluble in the ink carrier medium, such as water, so they are not easily washed away if a printed paper gets wet. Pigments also remain near the surface of the paper without diffusing outward as in the case of dye-based inks, so that edges of printed features are more well-defined. To provide higher contrast in printed images, pigment-based inks with high solids content are used together with a dispersant. To provide finer details in printed images, printheads having smaller nozzles are used in order to eject smaller drops. The qualities that can make the pigment-based inks desirable for printing, such as insolubility in the ink carrier medium, can make them more difficult to remove from the nozzles and nozzle face. The pigment particles can more easily clog small nozzles as volatile components of the ink evaporate. In addition, the dispersant in the ink can form a film on the nozzle face that can make dust and debris stick to the nozzle face. Furthermore, specialty inks such as inks for functional printing of electronic components, or inks for 3D printing can have ink components that form residues that are difficult to remove.
A second development within the inkjet printing industry is the increased use of commercial printing. Commercial inkjet printers are capable of printing high volumes of pages at high printing throughput. The printheads are typically pagewidth printheads and are relatively expensive. Although the printheads can be replaced, replacement incurs additional costs for printhead components and system servicing. In addition, it causes undesirable downtime for the commercial printing system. Cleaning methods are needed that can effectively remove tenacious nozzle clogs and ink residue films without shortening printhead lifetime.
A variety of non-contact cleaning systems and methods have been disclosed in the prior art for cleaning the nozzle face of an inkjet printhead without physical contact of a wiper or blotter. U.S. Pat. No. 5,574,485 discloses a cleaning solution that is held within a cleaning nozzle by surface tension to form a meniscus that is caused to bulge into contact with the printhead nozzle face and form a bridge of cleaning solution. The cleaning solution is ultrasonically excited by a piezoelectric material immediately upstream of the cleaning nozzle to provide a high frequency energized liquid meniscus to facilitate viscous plug removal without having mechanical contact with the nozzle face. Vacuum nozzles are positioned near the cleaning nozzle to remove the deposited cleaning solution together with any ink dissolved therein.
U.S. Pat. No. 4,600,928 discloses an inkjet printing apparatus having a cleaning system where ink is supported near the nozzle, and ultrasonic cleaning vibrations are imposed on the supported ink mass. Such cleaning using the ink itself can be implemented with ink cross-flowing through the printhead cavity or in cooperation with a varying pressure differential to cause ink to oscillate inwardly and outwardly within the nozzles.
U.S. Pat. No. 4,970,535 discloses an inkjet printhead face cleaner that provides a controlled air passageway through an enclosure formed against the printhead face. Air is directed through an inlet into a cavity in a body. The body has a face that is placed in sealing contact against the printhead face. The air is directed through the cavity past the inkjet nozzles and out through an outlet. A vacuum source can be attached to the outlet to further seal the two faces together. A collection chamber is positioned below the outlet to facilitate disposing of removed ink.
U.S. Pat. No. 6,196,657 discloses a cleaning assembly that is disposed proximate the printhead surface for directing a flow of fluid along the surface and across at least one nozzle in order to clean contaminants from the surface and the at least one nozzle. The cleaning assembly has a cup that includes a cavity and surrounds the at least one nozzle. The cleaning assembly includes a valve system in fluid communication with the cavity for allowing a fluid flow stream consisting of alternating segments of at least one liquid cleaning agent from a liquid cleaning agent source and another element such as a gas from a gas source or a second liquid cleaning agent from a liquid cleaning agent source into the cavity.
U.S. Pat. No. 6,145,952 discloses a cleaning assembly disposed relative to the printhead surface or nozzle for directing a flow of fluid along the surface or across the nozzle to clean the particulate matter from the surface or nozzle. The cleaning assembly includes a septum disposed opposite the surface or nozzle for defining a gap therebetween. Presence of the septum accelerates the flow of fluid through the gap to introduce a hydrodynamic shearing force in the fluid. This shearing force acts against the particulate matter to clean the particulate matter from the surface or nozzle. A pump in fluid communication with the gap is also provided for pumping the fluid through the gap. As the surface or orifice is cleaned, the particulate matter is entrained in the fluid. A filter is provided to separate the particulate matter from the fluid. U.S. Pat. No. 6,513,903 discloses a self-cleaning printer with a printhead having an orifice plate defining an inkjet orifice, a cleaning orifice and a drain orifice. The orifice plate further defines an outer surface between the orifices. A source of pressurized cleaning fluid is connected to the cleaning orifice and a fluid return is connected to the drain orifice for storing used cleaning fluid. A cleaning surface is disposed adjacent to and separate from the outer surface to define a capillary fluid flow path from the cleaning orifice across the inkjet orifice and to the drain orifice.
U.S. Pat. No. 6,572,215 discloses a self-cleaning printhead including a printhead body having an outer surface defining an inkjet orifice. A source of pressurized cleaning fluid is provided to generate a flow of cleaning fluid at the outer surface during cleaning. A fluid drain is provided to receive the flow of cleaning fluid. A movable flow guide defines a flow path from the source of pressurized cleaning fluid along the outer surface and inkjet orifice and to the fluid drain. During cleaning, a translation drive moves the flow guide along a path that diverges from the flow path.
U.S. Pat. No. 6,511,155 discloses a cleaning apparatus for cleaning debris from orifices in an inkjet printhead nozzle plate. The cleaning apparatus includes a structure defining a cleaning cavity between two horizontally contacting rollers where cleaning liquid is loaded, agitated, and dynamically sealed in the cavity through the rotation of the rollers. A relative movement is also provided between the nozzle plate and the cleaning structure so that the nozzle plate can be positioned above the cleaning cavity with the rotating rollers. The nozzle plate is spaced a small distance from the flow of the cleaning liquid so that cleaning fluid fills the small distance. The flow causes the cleaning fluid to engage the nozzle plate and remove debris from the nozzle plate and nozzles. After the cleaning cycle has ended the cleaning fluid is discarded.
U.S. Pat. No. RE39,242 discloses a wet-wiping printhead cleaning system including a treatment fluid applicator that places treatment fluid on at least one of the printhead nozzle face and a wiper. Treatment fluid is applied before wiping the printhead by projecting treatment fluid through the atmosphere, thereby avoiding direct contact between the applicator and the nozzle face or the wiper. The treatment fluid lubricates the wiper so as to lengthen wiper service life and enhance wiping performance, and makes the accumulated residue more removable by wiping.
U.S. Pat. No. 7,798,598 discloses a nozzle cleaning unit that includes a wiping portion. The wiping portion is moved to adjust a gap between the wiping portion and a printhead. Contact cleaning or non-contact cleaning is selected at the time of cleaning. The wiper is more wettable than the nozzle face, which has an ink repellent coating. In non-contact cleaning the wiper is brought close enough to the nozzle face that ink on the nozzle face contacts the wiper and is drawn to the wiper. As a result, there is less frequent contact between the wiper and the nozzle face so that abrasion of the ink repellent coating is reduced.
U.S. Pat. No. 7,918,530 discloses an embodiment where an inkjet printhead is cleaned by two operations. A first operation is forcibly ejecting ink through the inkjet nozzles to clean nozzles that may be blocked or partially clogged. The forcible ejecting of ink also entrains debris from the nozzle face. A second operation is directing a stream of a pressurized cleaning fluid across a surface of the inkjet printhead. Dried ink and debris are loosened by the force and possibly the chemical composition of the stream and are removed from the nozzle face.
U.S. Pat. No. 7,344,231 discloses an embodiment in which a cleaning station includes a tray containing a solvent. A rotary cleaning blade in the cleaning station is soaked in the solvent and then rotates in order to scrape the outer surface of the printhead to unblock nozzles. The cleaning station also includes a resilient wiping blade that scrapes the outer surface of the printhead in order to wipe or dry the nozzles after passage of the cleaning blade and remove residual dirt.
Despite the previous advances in non-contact cleaning of nozzle faces of inkjet printheads, what is still needed are printing system designs and cleaning methods that employ cleaning fluids while preventing excessive mixing of cleaning fluid with ink in the ink supply. What is also needed are printing system designs and cleaning methods that direct air toward and across the nozzle face without depriming nozzles. What is further needed are cleaning station designs and cleaning methods having improved effectiveness in removing residual cleaning fluid and ink without contacting the nozzle face in regions where nozzles are located.